/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.master.procedure;

import org.apache.hadoop.hbase.util.AvlUtil.AvlIterableList;
import org.apache.yetus.audience.InterfaceAudience;

@InterfaceAudience.Private
public class FairQueue<T extends Comparable<T>> {

    private Queue<T> queueHead = null;
    private int size = 0;

    public boolean hasRunnables() {
        return size > 0;
    }

    public void add(Queue<T> queue) {
        // For normal priority queue, just append it to the tail
        if(queueHead == null || queue.getPriority() == 1) {
            queueHead = AvlIterableList.append(queueHead, queue);
            size++;
            return;
        }
        // Find the one which priority is less than us
        // For now only TableQueue and ServerQueue has priority. For TableQueue there are only a small
        // number of tables which have higher priority, and for ServerQueue there is only one server
        // which could carry meta which leads to a higher priority, so this will not be an expensive
        // operation.
        Queue<T> base = queueHead;
        do {
            if(base.getPriority() < queue.getPriority()) {
                queueHead = AvlIterableList.prepend(queueHead, base, queue);
                size++;
                return;
            }
            base = AvlIterableList.readNext(base);
        } while(base != queueHead);
        // no one is lower than us, append to the tail
        queueHead = AvlIterableList.append(queueHead, queue);
        size++;
    }

    public void remove(Queue<T> queue) {
        queueHead = AvlIterableList.remove(queueHead, queue);
        size--;
    }

    public Queue<T> poll() {
        if(queueHead == null) {
            return null;
        }
        Queue<T> q = queueHead;
        do {
            if(q.isAvailable()) {
                if(q.getPriority() == 1) {
                    // for the normal priority queue, remove it and append it to the tail
                    queueHead = AvlIterableList.remove(queueHead, q);
                    queueHead = AvlIterableList.append(queueHead, q);
                }
                return q;
            }
            q = AvlIterableList.readNext(q);
        } while(q != queueHead);
        return null;
    }
}
